First of all, make sure the foot that runs the width of the front base of the iMac is popped forward. This lifts the base clear of the desk and allows more cool air to enter. Second, I would suggest propping up the rear an inch or so with a small block of wood or suchlike to get even more clearance. This will also raise the top vent relative to the rest of the case, which ought to improve airflow some.
The slot-loading iMacs have an internal plastic frame which is far more fragile and susceptible to falling apart than the external polycarbonate shell. It's a different plastic, and much thinner. Alas, from my memory of taking them apart a/ that's the part that everything else clips to and b/ once you've taken the iMac apart, that internal plastic is as likely as not to crumble to dust in your hands.
Yes, that's the stuff :-/
None at all AFAIK. The plastics would have been specified by Apple, and IIRC the ones we get here (Australia) were made in Thaliand. They suffer from the same plastic degradation. Your best bet is to ensure that it has sufficient ventilation underneath as described above.